This invention relates to the production of synthetic polyamides and more particularly to a process and apparatus for producing polyamides by the polymerization of aqueous polyamide forming salt solutions in a continuous process.
The synthetic linear polyamides prepared in the practice of this invention are of the general type described in U.S. Pat. Nos. 2,071,250; 2,071,253; 2,130,523; and 2,130,948. The polymers there described are high molecular weight products which are generally crystalline in structure showing X-ray powder diffraction patterns in the massive state, and which are capable of being cold drawn into fibers showing by characteristic X-ray patterns molecular orientation along the fiber axis.
These polyamides, generally speaking, comprise the reaction product of a polyamide-forming composition in which the molecules are bifunctional and contain two amide-forming groups, each of which is complementary to an amide-forming group in other molecules in said composition.
These polyamides as defined above or as otherwise identified hereinafter can be obtained, for example, by self-polymerization of monoaminomonocarboxylic acids, or by reacting a diamine with a dibasic carboxylic acid in substantially equimolecular amounts, it being understood that reference herein to the amino acids, diamines, and dibasic carboxylic acids is intended to include the equivalent amide-forming derivatives thereof. Amide-forming derivatives of the amino acids include the ester, anhydride, amide, lactam, acid halide, N-formyl derivatives, carbamate, and nitrile in the presence of water. Amide forming derivatives of the dibasic carboxylic acids comprise the mono- and di-ester, the anhydride, the mono- and di-amide, acid halide and the following compounds in the presence of water: nitrile, cyanocarboxylic acid, cyanoamide and cyclic imide. Amide forming derivatives of the diamines include the carbamate, N-formyl derivative and the N,N'-diformyl derivative.
While the term polyamide is inclusive of all polymeric materials which contain recurring amido groups, the term nylon is now accepted as the generic expression for those linear polyamides which may be fabricated into fibers. As referred to hereinafter nylon 66 is the polyamide derived from the condensation of hexamethylene diamine and adipic acid, nylon 6 is the polyamide derived from E-caprolactam and 66/6 nylon copolymers are interpolymers of nylon 66 and nylon 6. The possible combinations of diamines and dibasic acids as well as amino acids suitable for condensation reactions is quite large, however, for the purposes of describing the invention nylon 66 and interpolymers or copolymers of 66/6 nylon are specifically illustrated.
Relative viscosity, as used herein, is the ratio of viscosity (in centipoises) at 25.degree. C. of a 8.4% by weight solution of polyamide in 90% formic acid (90% by weight formic acid and 10% by weight water) to the viscosity (in centipoises) at 25.degree. C. of the 90% formic acid alone.
Various processes for the continuous polymerization of aqueous polyamide forming salt solutions, including removal of the solvent water and the volatile by-products (mostly water) of the condensation reaction, have been disclosed.
In one continuous process, U.S. Pat. No. 2,361,717 to Taylor, an aqueous solution of the polyamide forming reactants, e.g., a diaminedicarboxylic acid salt, is supplied continuously to a reactor wherein the temperature-pressure conditions are such that formation of steam is prevented and a major portion of the salt converted to a polymer. The resulting reaction mass is then supplied continuously to a flash tube wherein temperatures sufficiently high for polymerization to continue are maintained with a gradual pressure reduction, thus, permitting the separation of water from the reaction mass as steam. Finally, the reaction mass is fed continuously to a heated finisher where the polymerization is completed to the extent desired.
In another continuous process, U.S. Pat. No. 2,689,839 to Heckert, an aqueous solution of the polyamide forming reactants is fed continuously through a vented jacketed vessel wherein the temperature-pressure conditions are such that a major portion of the water is removed as steam and a portion of the salt is converted to a polymer. Heckert teaches the absence of mixing in order to provide a concentration gradient in the salt solution between the inlet and the outlet of the jacketed vessel. The residual material from the jacketed vessel is fed continuously to a series of flash tubes of progressively increasing diameter and then to a heated finisher to complete the polymerization to the extent desired.
In other continuous processes, for example, in U.S. Pat. No. 3,218,297 to Sovereign and U.S. Pat. No. 3,296,217 to Tate, most of the solvent water is evaporated from the aqueous polyamide forming salt solution while the salt solution is moving as a thin annular film in a first heated zone. The major portion of the residual salt solution from the first heated zone is converted to low molecular weight polymer and most of the volatile by-products (mostly water) of the condensation reaction are evaporated from a thin annular film of the salt solution in a second heated zone at a polyamide forming temperature. The residual material from the second heated zone may then be fed to a flasher and/or a finisher to attain the desired extent of reaction.
In yet another continuous process, U.S. Pat. No. 3,185,672 to Clemo et al., a hot aqueous polyamide forming salt solution is pumped through a pressure tube at a polyamide forming temperature under a pressure at least sufficient to prevent the formation of steam and for a time such that approximately 23 to 44% of the salt is converted to low molecular weight polyamide. The resulting solution of partially reacted monomer is expanded adiabatically through a narrow orifice into a heated vessel maintained at the same temperature as the pressure tube and at substantially atmospheric pressure. The solvent water and the volatile by-products of the condensation reaction are vented through a rectifying column to recover diamine therefrom. The partially reacted monomer is then removed from the heated vessel for further polymerization in other equipment.
In these prior art processes, the removal of the major portion of the solvent water and the conversion of the major portion of the polyamide forming aqueous salt solution to low molecular weight polymer with the removal therefrom of the major portion of volatile by-products (mostly water) formed in the condensation reaction are carried out either in an unagitated zone or in separate zones with the result that expensive heat transfer facilities are required.
Surprisingly, it has now been found that the above disadvantages of the prior art processes may be overcome and the above steps carried out simultaneously, in one zone, in inexpensive facilities, by injecting a polyamide forming aqueous salt solution into a reaction zone having mixing and heating means adapted to achieve uniform mixing and rapid heat transfer; the reaction zone being maintained at a temperature and pressure adapted to allow simultaneously, the removal of solvent water and the polymerization of the major portion of the salt.